1. Field of the Invention
The present invention relates to a quartz-crystal oscillator using an inverting amplifier, and more particularly to a crystal oscillator in which an output level of a rectangular output signal is restricted.
2. Description of the Related Art
A quartz-crystal oscillator constituted by a quartz-crystal unit and an oscillating circuit employing the crystal unit has been widely used in a variety of electronic appliances as a reference source of frequency and time. There are many types of crystal oscillators. One of them is a crystal oscillator employing an inverting amplifier including a CMOS (Complementary Metal-Oxide-Semiconductor) circuit, and used, for example, as a reference signal source of a PLL (Phase-Locked Loop) circuit of a portable telephone.
FIG. 1 illustrates an example of a circuit constitution of a conventional crystal oscillator employing the CMOS inverting amplifiers. This crystal oscillator includes constituents roughly classified into two groups, i.e., oscillating stage 1 and buffering stage 2, which are integrated into an integrated circuit (IC) chip. The crystal oscillator is mounted on a printed wiring board and supplies an oscillating output to respective circuits, i.e., respective external circuits on this printed wiring board.
Oscillating stage 1 comprises a resonance circuit with crystal unit 3 made of, for example, a quartz-crystal blank of AT cut, and oscillating capacitors 4a and 4b disposed on opposite sides of crystal unit 3 and connected, respectively, to a reference potential. In the illustrated example, the reference potential corresponds to the ground potential. Inverting amplifier 6 for oscillation, which has feedback resistor 5, is provided for amplifying an oscillating signal of the resonance circuit. Input and output terminals of inverting amplifier 6 are connected to one and the other ends of crystal unit 3, respectively. Inverting amplifier 6 is connected to and driven by the electric power supply that is constituted by a first constant voltage source Vd1.
The oscillating frequency of the oscillating stage 1 depends on a resonance frequency of the resonance circuit, and is determined by an equivalent serial capacitance (a load capacitance) viewing from crystal unit 3.
The inverting amplifier employed in this circuit will be described with reference to FIG. 2.
The inverting amplifier is a so-called CMOS inverter circuit formed by P-channel MOS (P-MOS) FET (Field Effect Transistor) 21 and N-channel MOS (N-MOS) FET 22, which are in series connection. In this example, the drain of P-MOS FET 21 and the drain of N-MOS FET 22 are connected together, and the source of P-MOS FET 21 is connected to power supply Vd while grounding the source of N-MOS FET 22. Further, the gates of respective MOS FETs 21 and 22 are connected at a common point to which input signal Vin is impressed. Output signal Vout is derived from the commonly connected drains. In the described inverting amplifier, with every high-level and low-level half waves of the frequency amplitude signal, which is impressed to the gates as input signal Vin, P-MOS FET 21 and N-MOS FET 22 perform amplifying operations to generate output signal Vout of which the phase is reversed from that of input signal Vin.
At oscillating stage 1, due to the employment of the above-described inverting amplifier, the phases between the input and output signals differ from one another by approximately 180 degrees, so as to satisfy a condition for causing oscillation, so that as an oscillating output, a rectangular signal may be obtained. Usually, the amplitude level of this oscillating output is the same as that of power supply voltage Vd1.
Buffering stage 2 is constituted by inverting amplifier 7, which is identical with the above-described inverting amplifier 6 and is provided with feedback resistor 5A, and another inverting amplifier 8 connected to the output terminal of inverting amplifier 7. The output of inverting amplifier 8 corresponds to output Vo of this crystal oscillator. The output of oscillating stage 1 is supplied to the input terminal of inverting amplifier 7, via capacitor 9 for blocking a direct current component. These inverting amplifiers 7 and 8 are connected to and driven by a power supply constituted by second constant voltage source Vd2. Voltage Vd2 of the second constant voltage source is set to be smaller than that Vd1 of the first constant voltage source.
Inverting amplifier 7 of buffering stage 2 is provided for restricting the amplitude of the oscillating output, that is, oscillating frequency signal, from oscillating stage 1 to a small value. The larger are the level or amplitude of the oscillating output at oscillating stage 1, the smaller is the noise, and from this viewpoint, the voltage of first constant voltage source Vd1 should preferably be made large. However, on the external circuit supplied with the oscillating signal from this crystal oscillator, if the level of the oscillating output is high, a current flowing through a load becomes large, and as a result, power consumption of those external circuit must become large. Therefore, it is preferred that the amplitude of the oscillating signal supplied to the external circuit is smaller. Thus, in this crystal oscillator, second constant voltage source Vd2 ( less than Vd1) is used for clipping the amplitude of the oscillating output so as to restrict it to a smaller value. Inverting amplifier 8 is provided for electrically shielding oscillating stage 1 from the external circuits, so that any unfavorable affect on oscillating stage 1 from the external circuits is made smaller.
Nevertheless, the above constituted crystal oscillator employing second constant voltage source Vd2 in buffering stage 2 must suffer from the fact that when it is formed in an IC chip, the chip area must necessarily increase while preventing the crystal oscillator from being downsized. Further, in buffering stage 2, although the oscillating output is clipped by second constant voltage source Vd2 due to feedback amplification done by inverter 7, such feedback amplification increases noise, and becomes a factor in worsening of the phase noise characteristic.
An object of the present invention is to provide a crystal oscillator, capable of promoting the downsizing thereof when it is formed in an IC chip and having a good phase noise characteristic.
The above object can be achieved by a crystal oscillator which comprises an oscillating stage which has a resonance circuit including a crystal unit and a capacitor, an inverting amplifier, and a feedback resistor connected to the inverting amplifier, the inverting amplifier feeding back and amplifying a resonance frequency component of the resonance circuit to output the resonance frequency component; and a buffering stage for outputting an output of the oscillating stage after reducing an amplitude thereof, the buffering stage including first and second MOS FETs of the same conductive type arranged in series between an electric power supply and a reference potential, wherein a signal at an output terminal of the inverting amplifier is impressed to a gate of said first MOS FET, a signal at an input terminal of the inverting amplifier is impressed to a gate of the second MOS FET, and an output is obtained from a connecting point of the first and second MOS FETs.
In the crystal oscillator according to the present invention, the first and second MOS FETs of the same conductive type are connected in series in a manner such that a pair of signals of reverse polarities from the oscillating stage are impressed to the gates of these MOS FETs, and an output is obtained from the connecting point of these MOS FETs. As a result, due to a threshold voltage between the gate and a source of the first MOS FET, the level of the output at the connecting point can be small. Therefore, the amplitude of the signal outputted from the buffering stage can be restricted to a small value while permitting the oscillating and buffering stages to be connected to a common power supply, and accordingly when the crystal oscillator is formed in an IC chip, the size thereof can be made small. Further, since the buffering stage does not employ any inverting amplifier having a feedback resistor, it is possible to remove a noise source, so that the phase noise characteristic can be improved.